Why are clouds white?

    Clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths (red, orange, yellow, green, blue, indigo, and violet), which combine to produce white light. Just like when you see a crystal with a light shinning through it and many colors come out the other side.
  Just because a cloud is dark doesn't meen that it is going to rain. Some clouds are dark because the top of the cloud is casting a shadow on it's base others can be because they are in another clouds shadow. A cloud turns gray when it is about to produce rain (in most situations).
  What is the worlds largest cloud ever spotted? The largest cloud ever reported was a toxic cloud that occurred in 1991. It reached 181 miles!